The present invention relates to a color image forming apparatus. This color image forming apparatus may be a copying machine, a printer, and a facsimile. The color image forming apparatus may utilize the technology of electro-photography, electro-statics, or may utilize some other technology for forming the image on a paper.
Conventionally, a tandem type of color image forming apparatus is known. In this conventional color image forming apparatus, a separate image forming unit for forming an image of a separate color, for example, yellow (hereafter Y), magenta (hereafter M), cyan (hereafter C), and black (hereafter K), respectively, are lined up along a conveyer belt. The conveyer belt conveys the paper, on which the image is to be formed, under each image forming unit. The conventional color image forming apparatus is shown in detail in FIG. 2.
As shown in FIG. 2, in the conventional color image forming apparatus, a plurality of image forming units 1Y, 1M, 1C, and 1K are sequentially disposed along a conveyer belt 3. The conveyer belt 3 is installed over conveyer rollers 4 and 5 so as to move in a direction shown by an arrow in FIG. 2. One roller of the conveyer rollers 4 and 5 also drives the conveyer belt 3 in addition to just moving the conveyer belt 3, and the other roller just moves the conveyer belt 3. A paper feeder comprises a paper feed tray 6. A pile of papers is placed in the paper feed tray 6. A paper 2 on the top of this pile of paper is fed to the conveyer belt 3 by not shown a paper feed roller. The paper 2 sticks to the conveyer belt 3 due to electrostatic adsorption.
The conveyer belt 3 conveys the paper 2 under the Y image forming unit 1Y. At this position, a transfer unit 7Y of the Y image forming unit 1Y forms a Y image (i.e. a yellow color image) on the paper 2. The Y image forming unit 1Y has a drum like photosensitive body 8Y that supports an image, a charging unit 9Y disposed in the periphery of the photosensitive body 8Y for uniformly charging the photosensitive body 8Y, an exposure unit 10Y as an exposure means performing exposure for the photosensitive body 8Y, a development unit 11Y as a development means developing an electrostatic latent image on the photosensitive body 8Y to form a Y toner image, and a photosensitive body cleaner 12Y as a cleaning means cleaning the photosensitive body 8Y.
The photosensitive body 8Y is rotated and driven by a not shown rotation mechanism so as to be charged uniformly by the charging unit 9Y. Then, the photosensitive body 8Y is exposed to a laser light 13Y from the exposure unit 10Y so as to form an electrostatic latent image. This electrostatic latent image on the photosensitive body 8Y is developed by the development unit 11Y so that the Y toner image is formed on the photosensitive body 8Y. This Y toner image on the photosensitive body 8Y is transferred onto the transfer paper 2 on the conveyer belt 3 at a position (a transfer position) where the photosensitive body 8Y and the transfer paper 2 on the conveyer belt 3 come in contact with each other by means of the transfer unit 7Y so that the Y toner image of only the yellow color is formed on the paper 2. After the transfer of the Y toner image is completed, the photosensitive body 8Y is prepared for the next image formation while unnecessary toner remaining on the surface is cleaned by the photosensitive body cleaner 12Y.
The paper 2 having the Y image formed thereon is then brought under the M image forming unit 1M by moving the conveyer belt 3. At this position, a transfer unit 7M of the M image forming unit 1M forms an M image (i.e. a magenta color image) over the already formed Y image. Then, the paper 2 having the Y image and M image formed thereon is brought under the C image forming unit 1C by moving the conveyer belt 3. At this position, a transfer unit 7C of the C image forming unit 1C forms an C image (i.e. a cyan color image) over the already formed Y image and M image. Then, the paper 2 having the Y image, M image, and C image formed thereon is brought under the K image forming unit 1K by moving the conveyer belt 3. At this position, a transfer unit 7K of the K image forming unit 1K forms an K image (i.e. a black color image) over the already formed Y image, M image, and C image. Thus, a full color image is formed on the paper 2. Then, the paper 2 having an image printed thereon is conveyed under a fixing unit 14. The fixing unit 14 fixes the image. Finally, the paper 2 is discharged outside the machine.
The M image forming unit 1M, the C image forming unit 1C, and the K image forming unit 1K have drum like photosensitive bodies 8M, 8C, and 8K as image support bodies, charging units 9M, 9C, and 9K disposed in the peripheries of the photosensitive bodies 8M, 8C, and 8K as charging means for uniformly charging the photosensitive bodies 8M, 8C, and 8K, exposure units 10M, 10C, and 10K as exposure means performing exposure for the photosensitive bodies 8M, 8C, and 8K, development units 11M, 11C, and 11K as development means developing electrostatic latent images on the photosensitive bodies 8M, 8C, and 8K to make an M toner image, a C toner image, and a K toner image, and photosensitive body cleaners 121M, 12C, and 12K as cleaning means cleaning the photosensitive bodies 8M, 8C, and 8K, respectively, similarly to the Y image forming unit 1Y.
In the M image forming unit 1M, the C image forming unit 1C, and the K image forming unit 1K, after the photosensitive bodies 8M, 8C, and 8K are rotated and driven by means of rotation mechanisms which are not shown so as to be uniformly charged with electricity by the charging units 9M, 9C, and 9K, respectively, they are exposed to laser lights 13M, 13C, and 13K corresponding to an M image signal, a C image signal, and a K image signal by the exposure units 10M, 10C, and 10K to form electrostatic latent images, respectively. These electrostatic latent images on the photosensitive bodies 8M, 8C, and 8K are developed by the development units 11M, 11C, and 11K so that the M toner image, the C toner image, and the K toner image are formed on the photosensitive bodies 8M, 8C, and 8K. The M toner image, the C toner image, and the K toner image on the photosensitive bodies 8M, 8C, and 8K are transferred onto the transfer paper 2 on the conveyer belt 3 at the positions (transfer positions) where the photosensitive bodies 8M, 8C, and 8K and the transfer paper 2 on the conveyer belt 3 come in contact with each other by means of the transfer units 7M, 7C, and 7K. After the transfer of the M toner image, the C toner image, and the K toner image are completed, the photosensitive bodies 8M, 8C, and 8K are prepared for the next image formations while unnecessary toner remaining on the surfaces is cleaned by the photosensitive body cleaners 12M, 12C, and 12K.
In the conventional color image forming apparatus it is necessary that the different color images are formed at exact positions. For example, if the positions of the image forming units is not aligned, then the image will get distorted. Therefore, a technique for an alignment between respective colors is a significant issue from the constitutional standpoint. The factors that cause static positional deviation of respective colors mainly include skew, positional deviation of the resist in the sub-scanning direction and main scanning direction, and error in magnification in the main scanning direction.
A skew correction is performed by, for example, regulating the inclination of an optical unit or the inclination of a beam reflection mirror existing therein or the like.
In the color image forming method described in Japanese Patent Application Laid-Open No. HEI 10-198110, detection unit for detecting a positional deviation detection mark formed by a line in a main scanning direction and a line inclined against that line is provided. This detection unit is composed of a slit having an aperture which is parallel to and has the same width as the respective lines of the mark, a light source, and a light receiver.
An image forming apparatus is disclosed in Japanese Patent No. 2642351 in which a standard part composed of a straight line extending on a non-end like conveying means in a main scanning direction and a slanting line extending in a slant manner against the straight line are formed, and a deviation amount of the slanting line in the main scanning direction is calculated through the comparison between an ideal value of the space between the standard part and the slanting line and an actually detected space, so that based on its result at least either of a main scanning write timing clock or write clock is corrected.
An image forming apparatus is disclosed in Japanese Patent No. 2765626 in which a pattern image for measurement is formed on a non-end like conveying means, this pattern image is detected with a detection unit, a deviation between a standard pattern image for one color is and the pattern image of other colors is measured, and the timing of forming the image is adjusted based on the measured deviation in which a pattern image for measuring positional deviations for respective colors is formed on a non-end like conveying means so that this pattern image for measuring is detected by a detection unit, and a deviation amount between a pattern image for measuring one color to be a standard and a pattern image for measuring another color is calculated so that based on the deviation amount, image write timing is regulated.
A multiple image forming apparatus is disclosed in Japanese Patent No. 2573855 in which reading of an alignment pattern for correcting a positional deviation for respective colors and reading of a density pattern for controlling the density of an image are performed by employing the same reading unit commonly.
FIG. 3 shows the detection unit and its peripheral section described in the Japanese Patent Application Laid-Open No. HEI 10-198110. FIG. 4 shows side and enlarged view of the detection unit. The detection unit is composed of a light emitter 15, a slit 16, and a light receiver 17 so as to detect the positional deviation detection mark 25 formed by image forming units 21 to 24 of respective colors on a conveyer belt 20 installed over conveyer rollers 18 and 19. This detection unit is disposed in both ends, respectively, in the main scanning direction, and corresponding to each detection unit, the positional deviation detection mark 25 is formed in both ends of the conveyer belt 20, respectively.
FIG. 5 shows the slit 16 in an enlarged manner. The slit 16 has an aperture 16a with a width xe2x80x9caxe2x80x9d which is parallel to a line parallel to the main scanning direction of the positional deviation detection mark 25 (hereafter, referred to as a horizontal line) and a line slanting against that horizontal line (hereafter, referred to as a vertical line) and a length xe2x80x9cbxe2x80x9d in order to detect the respective horizontal and vertical lines. A detection time difference of each line and a detection result of right/left are compared while taking black horizontal line as a standard in using the horizontal and vertical lines so that depending on the result, corrections of skew, a sub-scanning resist deviation, a main scanning resist deviation, and a main scanning magnification error are performed.
In the image forming apparatuses described in the Japanese Patent Nos. 2642351 and 2765626, a correction based on various kinds of deviation amounts calculated through detection results of the detection unit is performed as follows, similarly to the color image forming method described in the Japanese Patent Application Laid-Open No. HEI 10-198110.
FIG. 6 shows a timing chart of the time when the write timing of the sub-scanning direction is corrected. In this case, the correction resolution is supposes to be one dot. With respect to an image region signal (write enable signal) of the sub-scanning direction, the write position is regulated by the timing of a synchronized detection signal. Now, when it is wanted that the write position is advanced one dot depending on the result of the calculation using a detection result of the detection unit, the write enable signal may be made active one part fast of the synchronized detection signal as shown in FIG. 6.
FIG. 7 shows a timing chart of the time when the write timing of the main scanning direction is corrected. In this case, the correction resolution is supposes to be one dot. With respect to the image write clock, clocks having exactly matched phases can be obtained for respective lines by a rising edge of the synchronized detection signal. Writing of an image is performed, synchronizing that clock signal, and write enable signal of the main scanning direction is made, synchronizing that clock signal. Now, when it is wanted that the write position is advanced one dot depending on the result of the calculation using a detection result of the detection unit, the write enable signal may be made active one clock part fast as shown in FIG. 7.
Further, when the magnification in the main scanning direction is deviated from the standard color as a result of the calculation using a detection result of the detection unit by a color other than the standard color, the magnification of the main scanning direction can be altered by employing a device by which the frequency of the clock can be changed by a very small step, for example, a clock generator or the like.
In the multiple image forming apparatus described in the Japanese Patent No. 2573855, same detection unit detects the alignment mark and the density pattern (image density regulation pattern). In this case, the alignment mark (positional deviation detection mark) 25 shown in FIG. 3 and the image density regulation pattern 26 shown in FIG. 8 are formed individually on the conveyer belt 20 by the image forming units 21 to 24 of respective colors indifferent sequences.
A predetermined arithmetic processing is performed using the detection result of the alignment mark 25, and according to the result, the alignment control as described above is performed. Similarly, a predetermined arithmetic processing is performed using the detection result of the density pattern 26 for regulating an image density, and according to the result, a process condition or the like, such as a laser power, a charge bias, a development bias, or the like, is altered into an optimal value.
In the multiple image forming apparatus described in the Japanese Patent No. 2573855, since the alignment operation in which the detection result of the alignment mark 25 is calculated and in accordance with the result the alignment control is performed and the image density regulation operation in which the detection result of the density pattern 26 is calculated and in accordance with the result a process condition or the like is altered are executed in different sequences, useless time is taken so that a waiting time of a user becomes long.
It is the first object of the present invention to provide a color image forming apparatus by which a waiting time of a user can be shortened.
It is the second object of the present invention to provide a color image forming apparatus in which a running cost can be reduced.
It is the third object of the present invention to provide a color image forming apparatus by which a waiting time of a user can be shortened.
It is the fourth object of the present invention to provide a color image forming apparatus by which an alignment can be securely performed.
It is the fifth object of the present invention to provide a color image forming apparatus by which an alignment can be securely performed.
The color image forming apparatus according to one aspect of the invention comprises a means forming and detecting the positional deviation detection mark and the image density regulation mark. When requests for image density control and alignment are almost simultaneously received, this means forms and detects the positional deviation detection mark and the image density regulation mark in the sequence the requests were received. Therefore, the time for which the user is required to wait is shortened.
Further, it is preferable that if only a request for image density control is received only the image density regulation mark is detected and if only a request for alignment is received only the positional deviation detection mark is detected. Accordingly, the running cost can be reduced.
Further, it is preferable that the frequency for sampling a detection signal of the image density regulation mark and the frequency for sampling a detection signal of the positional deviation detection mark are different. Furthermore, it is preferable that the frequency for sampling the detection signal of the image density regulation mark is smaller than the frequency for sampling the detection signal of the positional deviation detection mark. Accordingly, the time for which the user is required to wait is shortened.
Further, it is preferable that a detection result of the positional deviation detection mark is discriminated. When the discrimination is impossible, only the image density regulation for each color is performed so that the alignment for each color is not performed. One the other hand, when the discrimination is possible, the image density regulation for each color and the alignment for each color are performed. Accordingly, alignment can be securely performed.
Further, it is preferable that, when the discrimination is impossible, the alignment for each color is performed after the image density regulation for each color is performed. Accordingly, alignment can be securely performed.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.